The present invention relates to an image forming apparatus that includes a developing unit for developing an electrostatic image using toner as a visualizing material, and is used to form an image corresponding to an object or an output signal, and also relates to an image forming method.
In developing units (developing methods) to be incorporated in an image forming apparatus using electrophotography, it is known that some employ a two-component developer containing a carrier and toner, and others employ a one-component developer containing only toner.
In the case of the two-component developing system, toner is attached to carrier particles and conveyed to the outer periphery of a developing sleeve, and a developer layer of a predetermined thickness is formed on the sleeve using a doctor blade. This developer layer is brought into contact with the surface of a photosensitive drum, thereby separating the toner from the carrier by the Coulomb force of an electrostatic latent image pre-formed on the photosensitive drum, and then applying the toner to the electrostatic latent image. Thus, the electrostatic latent image is developed.
In the case of the one-component developing system, a thin layer formed of only toner is formed on the outer periphery of the developing sleeve, and the photosensitive drum and the developing sleeve are opposed to each other with a predetermined space therebetween (or are put in contact with each other), thereby applying toner to an electrostatic latent image formed on the photosensitive drum to develop it.
At the present stage, although the phenomenon that occurs in a developing area, in which the developing sleeve and the photosensitive drum are opposed, accords to that occurring in the one-component developing system, a semi two-component developing system that uses a small amount of magnetic carrier is also proposed.
The semi two-component developing system is a developing system that uses, as a developer, a small amount of carrier (magnetic material) and a magnetic toner that contains magnetic material particles whose ratio to the toner is predetermined. In this method, a carrier thin layer made of a predetermined amount of carrier is formed on the developing sleeve and used to efficiently convey the magnetic toner to the developing area in which the photosensitive drum and the developing sleeve are opposed.
In the semi two-component developing system, at the present stage, a mixture of toner and a predetermined amount of magnetic material particles is widely used, as well as toner that contains magnetic material particles therein, in order to enhance the build up of the electrostatic charge characteristic of the toner. However, if an image forming process, i.e., printing output is repeated using the mixture of toner and magnetic material particles, the attached magnetic material particles may be separated from the surfaces of toner particles by stress, which is represented by the pressure applied between the developing sleeve and the doctor blade (toner layer thickness limiting member), pressure and friction resulting from a toner mixing mechanism and/or friction between toner particles. The magnetic material particles separated from the toner particle surfaces remain at a predetermined portion of the developing unit, and may cause white-background fogging (background fogging) on a non-imaging section (white-background section).